Abstract:

Purified SiHCl3 is used as a sweep gas across a permeate side of a gas
separation membrane receiving effluent gas from a polysilicon reactor.
The combined sweep gas and permeate is recycled to the reactor.

Claims:

1. A method for recycling effluent gas from a polysilicon production
reactor, comprising the steps of:directing an effluent gas from a
polysilicon reactor to a gas separation unit comprising at least one gas
separation membrane, said effluent gas comprising SiHCl3,
SiCl4, HCl, and H2;directing a sweep gas comprising high purity
SiHCl3 to a permeate side of the membrane;recovering a recycle gas
from the permeate side, the recycle gas comprising H2 permeated
through the membrane from the effluent gas and SiHCl3 from the sweep
gas; anddirecting the recycle gas to the polysilicon reactor.

2. The method of claim 1, further comprising the steps of:recovering a
retentate gas from the gas separation unit, the retentate gas comprising
SiHCl3, SiCl4, HCl, and H2;directing the retentate gas to
a SiHCl3 production process; andobtaining purified SiHCl3 from the SiHCl3
production process, wherein the sweep gas comprises at least a portion of
the obtained purified SiHCl3.

3. The method of claim 1, wherein the recycle gas is not compressed before
being directed to the polysilicon reactor.

4. The method of claim 1, wherein the effluent gas is not compressed
before being directed to the gas separation unit.

5. The method of claim 1, wherein at least 50% of H2 in the effluent gas
permeates to the permeate side.

6. The method of claim 1, wherein at least 90% of H2 in the effluent gas
permeates to the permeate side.

7. The method of claim 2, wherein said SiHCl3 production process comprises
the steps of:chilling the retentate gas to produce a first condensate and
a first non-condensate, the first condensate comprising predominantly
SiHCl3 and SiCl4, the first non-condensate comprising a major amount of
H2, a minor amount of chlorosilanes comprising SiHCl3, SiCl4, and a minor
amount of HCl;directing the first non-condensate to an adsorption unit
wherein the major amount of H2 is stripped and the minor amount of HCl is
separated from the minor amount of chlorosilanes;directing the first
condensate and the chlorosilanes to a distillation unit comprising at
least one distillation column; andproducing the purified SiHCl3 at the
distillation unit.

8. The method of claim 7, wherein said SiHCl3 production process further
comprises the steps of:feeding Si and the HCl separated from the
chlorosilanes to a first SiHCl3 reactor thereby producing impure
SiHCl3;purifying the impure SiHCl3 at a purification unit to produce a
SiHCl3 feed;directing the SiHCl3 feed to the distillation unit.

9. The method of claim 8, wherein said SiHCl3 production process further
comprises the steps of:feeding SiCl4 from the distillation unit, Si, and
H2 to a second SiHCl3 reactor in the presence of CuCl thereby producing
impure SiHCl3; andpurifying the impure SiHCl3 from the second SiHCl3
reactor at purification unit.

10. A system for recycling effluent gas from a polysilicon production
reactor, comprising:a gas separation unit comprising at least one gas
separation membrane, an inlet, a permeate outlet, and a retentate outlet,
the inlet being adapted and configured to fluidly communicate with an
effluent gas outlet of a polysilicon reactor, the permeate outlet being
adapted and configured to fluidly communicate with a reactant feed inlet
of the polysilicon reactor; anda SiHCl3 production unit adapted and
configured to produce purified SiHCl3 comprising an inlet in fluid
communication with the retentate outlet and an outlet in fluid
communication with a permeate side of the membrane.

11. The system of claim 10, wherein the membrane has a higher permeability
to H2 than SiHCl3, HCl, and SiCl4.

12. The system of claim 10, wherein there is no compressor in between the
permeate outlet and the reactant feed inlet.

13. The system of claim 10, wherein there is no compressor in between the
effluent gas outlet and the membrane inlet.

14. The system of claim 10, further comprising:a first condensation unit
having an inlet, a condensate outlet, and a vapor outlet, the
condensation inlet being in fluid communication with the retentate
outlet;an adsorption unit adapted and configured to strip H2 from a
SiCl4, SiHCl3, HCl, and H2 containing vapor from the vapor outlet of the
condensation unit and separate the remaining SiCl4, SiHCl3, and HCl into
HCl and chlorosilanes comprising the remaining SiCl4 and SiHCl3; anda
distillation unit having inlets in fluid communication with the first
condensation unit outlet and the adsorption unit and having a purified
SiHCl3 outlet in fluid communication with the inlet of the membrane.

15. The system of claim 14, wherein the first condensation unit comprises
first and second condensers connected in series and separated by a
compressor.

16. The system of claim 14, further comprising:a first SiHCl3 reactor
having reactant inlets in fluid communication with a source of Si and the
adsorption unit; anda purification unit having an inlet and outlet, the
purification unit inlet being in fluid communication with an outlet of
the first SiHCl3 reactor, the purification unit inlet being adapted and
configured to receive impure SiHCl3 from the first SiHCl3 reactor, the
purification unit outlet being in fluid communication with an inlet of
the distillation unit.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a Continuation-In-Part of U.S. patent
application Ser. No. 11/967,687 filed on Dec. 31, 2007, the entire
contents of which are incorporated herein by reference.

BACKGROUND

[0002]Effluent gas recovery from silicon production process is important
operation as it can reduce the cost of production. For a silicon
production using Siemens method, effluent gas leaving the deposition
reactor typically contains large quantities of hydrogen. This amount can
vary based on set of operating conditions.

[0003]Some have proposed to recycle some of the hydrogen from the effluent
gas using a gas separation membrane, such as disclosed by U.S. Pat. No.
4,941,893. However, such a solution comes with certain disadvantages.

[0004]When the deposition reactor is used for producing electronic grade
polysilicon, it is typically operated at a pressure of about 5 psig.
Hence, the product leaving the reactor does not offer much driving force
for separation using gas separation membranes. So, in order to obtain a
reasonable recovery, the effluent gas should be compressed prior to
feeding it to the membrane. Hydrogen being the fast gas will be recovered
in permeate stream operated at low pressure, for example, near deposition
reactor pressure.

[0005]On the other hand, when the deposition reactor is used to produce
solar grade polysilicon, it is typically operated at high pressure
(>75 psig). This means that the effluent gas leaving the reactor is at
a sufficiently high pressure to enable separation by gas separation
membrane. The relatively low pressure permeate stream will need to be
compressed to the deposition reactor pressure, thus adding to the
compressor cost.

[0006]Thus, it is an object to propose a method and system for effluent
gas recovery for polysilicon production that avoids the above described
disadvantages.

SUMMARY

[0007]There is disclosed a method for recycling effluent gas from a
polysilicon production reactor that includes the following steps. An
effluent gas from a polysilicon reactor is directed to a gas separation
unit that comprises at least one gas separation membrane, wherein the
effluent gas comprises SiHCl3, SiCl4, HCl, and H2. A sweep
gas comprising high purity SiHCl3 is directed to a permeate side of
the membrane. A recycle gas from the permeate side, wherein the recycle
gas comprises H2 permeated through the membrane from the effluent
gas and SiHCl3 from the sweep gas. The recycle gas is directed to
the polysilicon reactor.

[0008]The disclosed method may include one or more of the following
aspects: [0009]the method further comprises the steps of: [0010]a
retentate gas is recovered from the gas separation unit, the retentate
gas comprising SiHCl3, SiCl4, HCl, and H2; [0011]the
retentate gas is directed to a SiHCl3 production process; and
[0012]purified SiHCl3 is obtained from the SiHCl3 production process,
wherein the sweep gas comprises at least a portion of the obtained
purified SiHCl3. [0013]the recycle gas is not compressed before being
directed to the polysilicon reactor. [0014]the effluent gas is not
compressed before being directed to the gas separation unit. [0015]at
least 50% of H2 in the effluent gas permeates to the permeate side.
[0016]at least 90% of H2 in the effluent gas permeates to the permeate
side. [0017]the SiHCl3 production process comprises the steps of:
[0018]the retentate gas is chilled to produce a first condensate and a
first non-condensate, the first condensate comprising predominantly
SiHCl3 and SiCl4, the first non-condensate comprising a major amount of
H2, a minor amount of chlorosilanes comprising SiHCl3, SiCl4, and a minor
amount of HCl; [0019]the first non-condensate is directed to an
adsorption unit wherein the major amount of H2 is stripped and the minor
amount of HCl is separated from the minor amount of chlorosilanes;
[0020]the first condensate and the chlorosilanes are directed to a
distillation unit comprising at least one distillation column; and
[0021]the purified SiHCl3 is produced at the distillation unit.
[0022]the SiHCl3 production process further comprises the steps of:
[0023]Si and the HCl separated from the chlorosilanes are fed to a first
SiHCl3 reactor thereby producing impure SiHCl3; [0024]the impure SiHCl3
is purified at a purification unit to produce a SiHCl3 feed; [0025]the
SiHCl3 feed is directed to the distillation unit. [0026]the SiHCl3
production process further comprises the steps of: [0027]SiCl4 from the
distillation unit, Si, and H2 are fed to a second SiHCl3 reactor in the
presence of CuCl thereby producing impure SiHCl3; and [0028]the impure
SiHCl3 from the second SiHCl3 reactor is purified at purification unit.

[0029]There is also disclosed a system for recycling effluent gas from a
polysilicon production reactor that comprises: a gas separation unit and
a SiHCl3 production unit adapted and configured to produce purified
SiHCl3. The gas separation unit comprises at least one gas separation
membrane, an inlet, a permeate outlet, and a retentate outlet, wherein
the inlet is adapted and configured to fluidly communicate with an
effluent gas outlet of a polysilicon reactor and the permeate outlet is
adapted and configured to fluidly communicate with a reactant feed inlet
of the polysilicon reactor. The SiHCl3 production unit comprises an inlet
in fluid communication with the retentate outlet and an outlet in fluid
communication with a permeate side of the membrane.

[0030]The disclosed system may include one or more of the following
aspects: [0031]the membrane has a higher permeability to H2 than
SiHCl3, HCl, and SiCl4. [0032]there is no compressor in between the
permeate outlet and the reactant feed inlet. [0033]there is no compressor
in between the effluent gas outlet and the membrane inlet. [0034]the
system further comprises: [0035]a first condensation unit having an
inlet, a condensate outlet, and a vapor outlet, the condensation inlet
being in fluid communication with the retentate outlet; [0036]an
adsorption unit adapted and configured to strip H2 from a SiCl4, SiHCl3,
HCl, and H2 containing vapor from the vapor outlet of the condensation
unit and separate the remaining SiCl4, SiHCl3, and HCl into HCl and
chlorosilanes comprising the remaining SiCl4 and SiHCl3; and [0037]a
distillation unit having inlets in fluid communication with the first
condensation unit outlet and the adsorption unit and having a purified
SiHCl3 outlet in fluid communication with the inlet of the membrane.
[0038]the first condensation unit comprises first and second condensers
connected in series and separated by a compressor. [0039]the system
further comprises: [0040]a first SiHCl3 reactor having reactant inlets
in fluid communication with a source of Si and the adsorption unit;
[0041]a purification unit having an inlet and outlet, the purification
unit inlet being in fluid communication with an outlet of the first
SiHCl3 reactor, the purification unit inlet being adapted and configured
to receive impure SiHCl3 from the first SiHCl3 reactor, the purification
unit outlet being in fluid communication with an inlet of the
distillation unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]For a further understanding of the nature and objects of the present
invention, reference should be made to the following detailed
description, taken in conjunction with the accompanying drawings, in
which like elements are given the same or analogous reference numbers and
wherein:

[0043]FIG. 1 is a schematic of an embodiment of the system and method of
the invention.

[0044]FIG. 2 is a schematic of another embodiment of the system and method
of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0045]As best shown in FIG. 1, in one embodiment a trichlorosilane (TCS)
and H2 feed 3 from feedstock tank 1 are fed to polysilicon reactor 5
where they react according to the below reaction:

SiHCl3+H2→Si+3HCl

The following equilibrium reactions also play a role:

2 SiHCl3→Si+SiCl4+2HCl (1050-1200° C.)

4 SiHCl3→3SiCl4+2H2+Si

SiHCl3+HCl→SiCl4+H2

TCS is also in equilibrium with SiCl2, a key intermediate:

SiHCl3→SiCl2+HCl

While the schematic crudely depicts a bell jar shape, the invention is
equally applicable to Siemens-type bell jar reactors and fluidized bed
reactors. A wall temperature of the reactor is maintained at a
temperature of about 575° C. and a deposition temperature is
maintained at a temperature of about 1125° C. One of ordinary
skill in the art will recognize that the TCS and H2 need not be fed to
reactor from a feedstock tank 1. Rather, each of the reactants may be fed
directly to reactor 5 without the intermediary feedstock tank 1. If the
reactor 5 is being used to make electronic grade polysilicon, it is
typically operated at a pressure of about 5 psig. In the case of solar
grade polysilicon, it is operated at a pressure of 75 psig or greater.

[0046]Effluent gas stream 9 containing silicon tetrachloride (STC), an
amount of non-reacted TCS, HCl, and H2 is directed to gas separation unit
11 where it is separated into a H2-rich stream and a H2-lean
stream 12 containing TCS, STC, HCl, and a minor amount of H2. The gas
separation unit 11 includes one or more gas separation membranes. The
H2-lean stream 12 is directed to TCS purification system P which produces
purified TCS stream 89, H2 stream 7, and optional H2 stream 10. All or
part of the purified TCS for reaction in reactor 5 may be fed in stream
89 to the permeate "side" of the membrane where it acts as a sweep gas to
lower the partial pressure of H2 permeating through the membrane from the
effluent gas 9. One of ordinary skill in the art will recognize that the
permeate "side" of a membrane does not necessarily mean one and only one
side of a membrane. Rather, in the case of membranes include a plurality
of hollow fibers, the permeate "side" actually is considered to be the
plurality of sides of the individual hollow fibers that are opposite to
the sides to which the effluent gas 9 is introduced.

[0047]Optionally and in order to reduce the tendency of TCS in stream 89
to condense, H2 stream 10 may be added to stream 89 to lessen such
tendency. Also, one of ordinary skill in the art will recognize that any
portion of purified TCS not directed to gas separation unit 11 may be
directed to feedstock tank 1 instead.

[0048]The combined TCS sweep gas and H2-rich stream comprise the TCS/H2
recycle 94. Because the ratio of TCS to H2 in the combined recycle 94 and
stream 7 may not be equivalent to the stoichiometric ratio desired for
the reactor 5, one may optionally supplement the TCS and H2 in feedstock
tank 1 with optional make up TCS and optional make up H2 91. Also, if the
pressure of the recycle 94 is lower than that of the feedstock tank 1, an
optional compressor 8 may be used to boost its pressure to the desired
level.

[0049]Practice of the invention yields several benefits.

[0050]By using the purified TCS gas stream 89 (and optionally H2 stream
10) to sweep the permeate side of the membrane, the partial pressure of
H2 on the permeate side is decreased thus increasing the partial pressure
driving force for this separation. Almost all of the H2, preferably at
least 90%, can be transferred from the effluent side to permeate side by
using a membrane with a high enough selectivity for H2 over
chlorosilanes. However, one of ordinary skill in the art will recognize
that the invention may be performed such that as little as 50% of the H2
can be transferred from the effluent side to the permeate side.
Additionally, depending upon the pressure of the feedstock tank 1, the
resultant TCS and H2 recycle 94 may be at a pressure sufficiently
high enough that the recycle 94 need not be compressed by compressor 8
before being fed to feedstock tank 1. Alternatively, the degree of
compression by compressor 8 of the recycle 94 may be reduced in
comparison to conventional solutions not utilizing a TCS sweep gas. The
phrase "recycle gas is directed to the polysilicon reactor" is not
limited to methods whereby the recycle gas goes directly to the
polysilicon reactor. Also, practice of the disclosed method is not
limited to those whereby the recycle gas goes directly to the polysilicon
reactor. Rather, it is within the scope of the disclosed method and
claimed subject matter to include one or more intermediate vessels for
containing the recycle gas or buffer vessels for buffering a flow of
recycle gas to the reactor. One such vessel is the feedstock tank 1.

[0051]Since only the retentate flow from the gas separation unit 11 is fed
to the purification system P, the reduced mass flow rate enables the use
of smaller volume equipment and lowered energy requirements. Also, since
the H2 permeation rate is much faster than any other species present in
the effluent gas stream 9, the permeate stream has a negligible amount of
undesirable impurities.

[0052]Suitable gas separation membranes include those chemically resistant
to TCS, STC, H2, and HCl and which exhibit an enhanced permeance of H2 in
comparison to the TCS, STC, and HCl. Such membranes can be configured in
a variety of ways: sheet, tube, hollow fiber, etc.

[0053]Preferably, the gas separation membrane of gas separation unit 11 is
a spiral flat sheet membrane or hollow fiber membrane made of a polymeric
material such as a polysulfone, a polyether sulfone, a polyimide, a
polyaramide, a polyamide-imide, and blends thereof.

[0054]One preferred type of hollow fiber membrane includes those disclosed
by U.S. Published Patent Application 2006/0156920 A1, the contents of
which are enclosed herein in their entirety. Those hollow polymeric
fibers include polyimides, polyamides, polyamide-imides, and blends
thereof. They include an outer selective layer.

[0055]The polyimide contains the repeating units as shown in the following
formula (I):

##STR00001##

in which R1 of formula (I) is a moiety having a composition selected
from the group consisting of formula (A), formula (B), formula (C), and
mixtures thereof, and

##STR00002##

in which R4 of formula (I) is a moiety having a composition selected
from the group consisting of formula (Q), formula (S), formula (T) and
mixtures thereof,

##STR00003##

in which Z of formula (T) is a moiety selected from the group consisting
of formula (L), formula (M), formula (N) and mixtures thereof.

##STR00004##

In one preferred embodiment, the polyimide component of the blend that
forms the selective layer of the membrane has repeating units as shown in
the following formula (Ia):

##STR00005##

In this embodiment, moiety R1 of formula (Ia) is of formula (A) in
0-100% of the repeating units, of formula (B) in 0-100% of the repeating
units, and of formula (C) in a complementary amount totaling 100% of the
repeating units. A polymer of this structure is available from HP Polymer
GmbH under the trade name P84. P84 is believed to have repeating units
according to formula (Ia) in which R1 is formula (A) in about 16% of
the repeating units, formula (B) in about 64% of the repeating units and
formula (C) in about 20% of the repeating units. P84 is believed to be
derived from the condensation reaction of benzophenone tetracarboxylic
dianhydride (BTDA, 100 mole %), with a mixture of 2,4-toluene
diisocyanate (2,4-TDI, 64 mole %), 2,6-toluene diisocyanate (2,6-TDI, 16
mole %) and 4,4'-methylene-bis(phenylisocyanate) (MDI, 20 mole %).

[0056]The polyimide (that is preferably formed in a known way to provide
an outer selective layer) comprises repeating units of formula (Ib):

##STR00006##

[0057]In one preferred embodiment, the polyimide is of formula (Ib) and
R1 of formula (Ib) is a composition of formula (A) in about 0-100%
of the repeating units, and of formula (B) in a complementary amount
totaling 100% of the repeating units.

[0058]In yet another embodiment, the polyimide is a copolymer comprising
repeating units of both formula (Ia) and (Ib) in which units of formula
(Ib) constitute about 1-99% of the total repeating units of formulas (Ia)
and (Ib). A polymer of this structure is available from HP Polymer GmbH
under the trade name P84HT. P84HT is believed to have repeating units
according to formulas (Ia) and (Ib) in which the moiety R1 is a
composition of formula (A) in about 20% of the repeating units and of
formula (B) in about 80% of the repeating units, and, in which repeating
units of formula (Ib) constitute about 40% of the total of repeating
units of formulas (Ia) and (Ib). P84HT is believed to be derived from the
condensation reaction of benzophenone tetracarboxylic dianhydride (BTDA,
60 mole %) and pyromellitic dianhydride (PMDA, 40 mole %) with
2,4-toluene diisocyanate (2,4-TDI, 80 mole %) and 2,6-toluene
diisocyanate (2,6-TDI, 20 mole %). The polyamide polymer of the blend
that forms the selective layer of the membrane comprises the repeating
units of the following formula (II):

##STR00007##

in which Ra is a moiety having a composition selected from the group
consisting of formulas

##STR00008##

wherein Z' of formula (g) is a moiety represented by the formula

##STR00009##

and mixtures thereof, andin which X, X1, X2, and X3 of
formulas a, b, d, e, f, g, h, j, and, l independently are hydrogen or an
alkyl group having 1 to 6 carbon atoms, and Z'' of formula (I) is
selected from the group consisting of:

##STR00010##

in which X of formula (p) is a moiety as described above.

[0059]R2 of formula (II) is a moiety having a composition selected
from the group consisting of formulas:

##STR00011##

[0060]and mixtures thereof.

[0061]The polyamide-imide polymers of the blend that forms the selective
layer of the membrane comprises the repeating units of formula (III);
and/or a combination of the repeating units of formulas (I) and (II), (I)
and (III), (II) and (III), and/or (I), (II), and (III).

##STR00012##

in which Ra, R2, and R4 are the same as described above,
and

R3 is

##STR00013##

[0063]Membranes made from a blend of a polyimide or polyimides with a
polyamide or polyamides, the ratio of polyimide to polyamide should
preferably be at least 1:1, and more preferably, at least 2:1.

[0064]In the case of membranes made from a blend of a polyimide or
polyimides with a polyamide-imide or polyamide-imides, the ratio of
polyimide to polyamide-imide should preferably, be at least 1:1, and more
preferably at least 2:1.

[0065]In the case of membranes made from a blend of a polyimide or
polyimides with a polyamide or polyamides, and a polyamide-imide or
polyamide-imides, the blend should preferably contain between 20-80%
polyimide.

[0066]Surprising, the blends of this invention are homogeneous over a
broad range of compositions. The miscibility of the blends of this
invention may be confirmed by the presence of single compositional
dependent glass transition temperature lying between those of the
constituent blend components. The glass transition temperature can be
measured by Differential Scanning Calorimetry or Dynamic Mechanical
Analysis.

[0067]The polyimides described above are made by methods well known in the
art. The polyimides can, for example, be conveniently made by
polycondensation of an appropriate diisocyanate with approximately an
equimolar amount of an appropriate dianhydride. Alternatively, the
polyimides can be, for example, made by polycondensation of equimolar
amounts of a dianhydride and a diamine to form a polyamic acid followed
by chemical or thermal dehydration to form the polyimide. The
diisocyanates, diamines, and dianhydrides useful for making the
polyimides of interest are usually available commercially. The polyimides
are typically prepared by the latter diamine process because the diamines
are more readily available than the corresponding diisocyanates.

[0068]The polyamides described above can be made conveniently by
polycondensation of an appropriate diamine or diamines with approximately
an equimolar amount of an appropriate diacid chloride or mixtures of
diacid chlorides by methods well known in the art.

[0069]The polyamide-imide polymers described above can be made
conveniently by polycondensation of an appropriate diamine with
approximately an equimolar amount of an appropriate triacid
anhydride/chloride (i.e., repeating units of formula (III)).

[0070]In the case of a mixture of polyamide/polyamide-imides, the
polyamide-imides described herein can be made conveniently by: [0071]1)
polycondensation of an appropriate diamine or diamines with an equimolar
amount a mixture of dianhydride and diacid chloride mixture (i.e.,
repeating units of formulas (I) and (II)); [0072]2) by polycondensation
of an appropriate diamine or diamines with an equimolar amount of a
mixture of dianhydride and triacid anhydride chloride (i.e., repeating
units of formulas (I) and (III)); [0073]3) by polycondensation of an
appropriate diamine or diamines with an equimolar amount of a mixture of
diacid-chloride and triacid anhydride/chloride (i.e., repeating units of
formulas II and III); or [0074]4) by polycondensation of an appropriate
diamine or diamines with an equimolar amount of a mixture of dianhdride,
diacid chloride, and triacid anhydride/chloride (i.e., repeating units of
formulas I, II, and III).

[0075]The polyimides, polyamides, and polyamide-imides should be of
suitable molecular weight to be film forming and pliable so as to be
capable of being formed into continuous films or membranes. The polymers
of this invention preferably have a weight average molecular weight
within the range of about 20,000, to about 400,000, and more preferably,
about 50,000 to about 300,000.

[0076]Another type of polymeric material particularly useful in the
membrane includes an amorphous polymer of
perfluoro-2,2-dimethyl-1,3-dioxole, as disclosed in U.S. Pat. No.
5,051,114, the contents of which are incorporated herein in their
entirety. It may be a homopolymer of perfluoro-2,2-dimethyl-1,3-dioxole.
It may instead be a copolymer of perfluoro-2,2-dimethyl-1,3-dioxole,
including copolymers having a complementary amount of at least one
monomer selected from the group consisting of tetrafluoroethylene,
perfluoromethyl vinyl ether, vinylidene fluoride and
chlorotrifluoroethylene. Preferably, the polymer is a dipolymer of
perfluoro-2,2-dimethyl-1,3-dioxole and a complementary amount of
tetrafluoroethylene, especially such a polymer containing 65-99 mole % of
perfluoro-2,2-dimethyl-1,3-dioxole. The amorphous polymer preferably has
a glass transition temperature of at least 140° C., and more
preferably at least 180° C. Examples of dipolymers are described
in further detail in U.S. Pat. No. 4,754,009, the contents of which are
incorporated herein in their entirety.

[0077]Another type of polymeric material particularly useful in the
membrane includes a polymer available under the trade name MATRIMID 5218,
a polymer available under the trade name ULTEM 1000, and blends thereof
as disclosed in U.S. Pat. No. 5,248,319. MATRIMID 5218 is the polymeric
condensation product of 3,3',4,4'-benzophenone tetracarboxylic
dianhydride and 5(6)-amino-1-(4'-aminophenyl)-1,3,3'-trimethylindane,
commercially available from Ciba Specialty Chemicals Corp. Ultem 1000 may
be obtained from a wide variety of commercial sources, including Polymer
Plastics Corp. located in Reno, Nev. and Modern Plastics located in
Bridgeport, Conn. Ultem 1000 has the formula shown below.

##STR00014##

[0078]The membranes of the invention typically have continuous channels
for fluid flow extending between the exterior and interior surfaces.
These pores have an average cross-sectional diameter less than about
20,000 Angstroms, preferably less than about 1,000 or 5,000 Angstroms.
The hollow fibers may have outside diameters of about 20 to 1,000
microns, generally about 50 to 1,000 microns, and have walls of at least
about 5 microns in thickness, generally about 50 to about 1,000 microns
thick. The wall thickness in some hollow fibers may be up to about 200 or
300 microns. The coating may have a thickness ranging from about 0.01 to
about 10 microns and preferably has a thickness of about 0.05 to about 2
microns.

[0079]In the case of hollow fiber membranes, in order to provide desirable
fluxes through the hollow fibers, particularly using those hollow fibers
having walls at least about 50 microns in thickness, the hollow fibers
may have a substantial void volume. Voids are regions within the walls of
the hollow fibers which are vacant of the material of the hollow fibers.
Thus, when voids are present, the density of the hollow fiber is less
than the density of the bulk material of the hollow fiber. Often, when
voids are desired, the void volume of the hollow fibers is up to about
90, generally about 10 to 80, and sometimes about 20 or 30 to 70, percent
based on the superficial volume, i.e., the volume contained within the
gross dimensions, of the hollow fiber or flat sheet.

[0080]The density of the hollow fiber can be essentially the same
throughout its thickness, i.e., isotropic, but the hollow fiber is
preferably characterized by having at least one relatively dense region
within its thickness in barrier relationship to fluid flow through the
wall of the hollow fiber, i.e., the hollow fiber is anisotropic.

[0081]One of ordinary skill in the art will recognize that well known
system parameters such as the number of fibers can be adjusted such that
recycle 94 leaving the permeate side of the membrane has a composition
suitable for the deposition reactor.

[0082]As best illustrated in FIG. 2, in another embodiment further details
regarding the TCS purification are described. TCS and H2 feed 3 from
feedstock tank 1 are fed to polysilicon reactor 5 where they react
according to the below reactions:

SiHCl3+H2→Si+3HCl

The following equilibrium reactions also play a role:

2 SiHCl3→Si+SiCl4+2HCl (1050-1200° C.)

4 SiHCl3→3SiCl4+2H2+Si

SiHCl3+HCl→SiCl4+H2

TCS is also in equilibrium with SiCl2, a key intermediate:

SiHCl3→SiCl2+HCl

While the schematic crudely depicts a bell jar shape, the invention is
equally applicable to Siemens-type bell jar reactors and fluidized bed
reactors. A wall temperature of the reactor is maintained at a
temperature of about 575° C. and a deposition temperature is
maintained at a temperature of about 1100° C. One of ordinary
skill in the art will recognize that the TCS and H2 need not be fed to
reactor 5 from a feedstock tank 1. Rather, each of the reactants may be
fed directly to reactor 5 without the intermediary feedstock tank 1.
Because the ratio of TCS to H2 in recycle 94 may not be equivalent to the
stoichiometric ratio desired for the reactor 5, one may optionally
supplement the TCS and H2 in feedstock tank 1 with optional make up TCS
and optional make up H2 91.

[0083]Effluent gas stream 9 containing silicon tetrachloride (STC), an
amount of non-reacted TCS, HCl, and H2 is directed to gas separation
membrane 11 where it is separated into a H2-rich stream typically
containing about 93% by volume H2 and a H2-lean stream 12 containing
TCS, STC, HCl, and a minor amount of H2.

[0084]H2-lean stream 12 is condensed at a temperature of about -40°
C. and -60° C. in two stages at condensers 13, 21, respectively,
with intermediate compression at compressor 17 to a pressure of about 153
psig. The condensates 15, 23 contain mixtures of TCS, STC, and dissolved
HCl while the vapor component 24 contains a mixture of TCS, STC, HCl, and
H2.

[0090]Purified TCS stream 89 is fed to the permeate "side" of the membrane
where it acts as a sweep gas to lower the partial pressure of H2
permeating through the membrane from the effluent gas 9. One of ordinary
skill in the art will recognize that the permeate "side" of a membrane
does not necessarily mean one and only one side of a membrane. Rather, in
the case of membranes include a plurality of hollow fibers, the permeate
"side" actually is considered to be the plurality of sides of the
individual hollow fibers that are opposite to the sides to which the
effluent gas 9 is introduced.

[0091]Optionally and in order to reduce the tendency of TCS in stream 89
to condense, H2 stream 10 may be added to stream 89 to lessen such
tendency. Also, one of ordinary skill in the art will recognize that any
portion of purified TCS not directed to gas separation unit 11 may be
directed to feedstock tank 1 instead. Thus, the combined TCS (and
optional stream 10 H2) sweep gas and H2-rich stream comprise the TCS/H2
recycle 94.

[0092]Using TCS stream 89 (and optional H2 stream 10) on the permeate side
of the membrane maintains a high driving force for recovering H2. Because
of this, TCS can be fed at high pressure without sacrificing the
separation. In one example case simulated using membrane simulation tool,
a TCS sweep gas could be operated at 45 psig while recovering >90% of
hydrogen. In practical terms, the sweep stream pressure can even be
higher, even up to feed pressure of effluent stream. The only drawback of
such a high pressure would be the increased permeation of HCl. This can
be, however, fixed by choosing operating conditions/membrane such that a
high H2/HCl selectivity could be obtained. Also, if the pressure of
the recycle 94 is lower than that of the feedstock tank 1, an optional
compressor 8 may be used to boost its pressure to the desired level.

[0093]Preferred processes and apparatus for practicing the present
invention have been described. It will be understood and readily apparent
to the skilled artisan that many changes and modifications may be made to
the above-described embodiments without departing from the spirit and the
scope of the present invention. The foregoing is illustrative only and
that other embodiments of the integrated processes and apparatus may be
employed without departing from the true scope of the invention defined
in the following claims.